This invention relates to earth formation drilling and drilling hydraulics, and more particularly, to jet bits and nozzles for jet assisted drilling.
Rotary drill bits are used in the drilling of deep holes such as oil wells. Some are polycrystalline diamond compact ("PDC") bits with segmented rows or sectors of diamond hardened cutters; others are rotary cone drill bits. Other types of drill bits can be natural diamond, rock bits, underreamers and coring tools. The rotary cone bits have a plurality of rotating toothed conical cutters with vertices directed toward the centerline of the drill bit. The conical cutters are rotatively borne upon cantilevered journal shafts which extend from the lower periphery of the bit body angularly downward and radially inward relative to the centerline of the vertically cylindrical bit body. In each bit, the bit body upper end is threaded for attachment to the lower end of a drill line made of pipe. In normal drilling operations, the drill line pipe is rotated while forcing the rock bit into the earth. The sectors of teeth in a PDC bit or the cones in a rotary cone bit travel about the centerline of the drill bit and the rock cutters dig into the geologic formation to fail scrape, crush and/or fracture it.
The bit body also serves the function of a terminal pipe fitting to control and route a drilling fluid flow from inside the drill line pipe out through a plurality of mud nozzles housed in the drill bit and up the annulus between the drill column and the well bore. The drilling fluid accomplishes a number of critically important tasks, the foremost of which is preventing formation fluids from entering the well bore and causing a blowout. Drilling fluids ("muds") are weighted to provide a hydrostatic pressure in the well bore at any given depth that at least equals the formation pressure at the particular depth. Mud weights are usually controlled by adding a high density material such as barite to the mud. Drilling muds are thixotropic fluids that have high viscosity's at low shear rates and low viscosity's at high shear rates. At the high shear rates in bit nozzles, the mud has plastic flow characteristics approaching Newtonian behavior, like water. Jetted from the bit nozzles, it is employed to dissipate the heat of drilling and to flush cuttings from the drilling zone. At the lower shear rates in the annulus between the well bore wall and the drill line pipe, the viscosity increases and is sufficient to buoy cuttings upward to the surface for filtering from the mud. Vertical channels, sometimes called "junk slots," are formed between the exterior wall of the rock bit body adjacent the nozzle locations and the bore hole wall to facilitate the flow of fluid and entrained cuttings from the drilling zone.
Cuttings removal is critically important to the rate of penetration of the drilled formation, for control of viscosity of the drilling fluid, and to minimize wear and tear on drilling rig mud circulation apparatus. Inadequate removal of cuttings from the interface between the cutters of the drill bit and the formation rock causes the more substantial rock chips on the hole bottom to be ground to a paste by the bit. For example, a cube of particle 200 microns on each side, if allowed to remain in the bore hole, could be ground into eight million one micron cubes. These cuttings, called "drilled solids," approach colloidal size and hydrate in the fluid, increasing fluid viscosity at the bit ("plastic viscosity"). As plastic viscosity of the mud increases, drilling rate decreases. This is because the mud must get under a chip quickly so the bit cutters do not grind the chip instead of formation rock. If viscosity is high, the fluid cannot get under the chip rapidly and efficiently flush cuttings from the hole bottom. This impedes the penetration of the rock bit into the geological formation, abrasively wears the cutters of the rock cutters, causes excessive drag, and can produce well bore damage. If the drilled solids are left in the mud, the viscosity of the mud in the annulus increases and can make thick filter cakes that reduce the area for moving mud up the annulus. This can lead to lost circulation and formation damage and to stuck drill pipe.
The prior art has recognized that the pressure differential between the drilling fluid and the formation fluid hinders efficient removal of cuttings from the bore hole bottom and reduces rate of penetration. Various techniques are used to make the fluid emerging from the bit nozzles clean the bottom of the hole. One is to try to make the fluid hit the hole bottom as hard as possible; this is called optimizing hydraulic impact. Another is to try to make the fluid expend as much power across the nozzles as possible; this is called optimizing hydraulic horsepower.
The conventional mud nozzle in the drilling bit is an axially symmetrical, usually circular orifice. Typically a plurality of nozzles are employed. In a PDC bit the jets are spaced in front of the leading edge of a row or sector of teeth, and in a rotary cone bit, a nozzle is provided for each rotary rock cutter, positioned to direct a high velocity fluid stream downward between cutters and against the well bore wall to wash the face of the cutter cones and flush cuttings to the annulus. Generally the stream fans out substantially conically after leaving the nozzle. However, use of these high pressure nozzles for injecting drilling fluid into the bore hole has not satisfactorily provided the desired efficient removal of rock chips to the annulus and the vertical chip channels in the bit body. If the high velocity fluid stream reaches the entrance to the junk channels, the force of the stream can even hinder fluid flow up the channel, exacerbating the pressure differential hold down effect on formation cuttings. Substantial effort has been directed to this continuing problem of cuttings removal and bit balling.
It is also known that turbulent pressure fluctuations have been found to provide lifting forces sufficient to overcome rock chip holddown to remove rock debris from the hole bottom. This technique eases the work of the drill bit itself and facilitates drilling of the well bore.
U.S. Pat. No. 2,901,223 by Scott, proposes a centrally located cluster of three nozzles to discharge radially outward and downward between cutters which are relatively smaller than commonly used to avoid excessive abrasion from the nozzle discharge.
Johnson, in U.S. Pat. No. 3,528,704 and in U.S. Pat. No. 3,713,699 teaches the use of cavitating nozzles directly as cutting tools against the rock. A fluid stream is pulsated at high frequency and enough energy to physically vaporize the fluid in the low pressure phases of the vibratory wave. The vapor bubbles thus produced implode in the high pressure phases of the same waves, and, if very close to the rock surface, cause particles of the rock to erode away in tension. Later variations are described in U.S. Pat. Nos. 4,262,757 and 4,391,339 also to Johnson and in 4,378,853 to Chia.
Hayatdavoudi, in U.S. Pat. Nos. 4,436,166 and 4,512,420, includes a nozzle in a drilling sub above the drilling bit. The nozzle is oriented to eject drilling fluid from the sub into the annulus above the bit with a horizontal velocity component tangential to the annulus, to impart a swirling motion to the drilling fluid in the annulus and create a vortex supposed to suck cuttings radially outward from the cutter formation interface and upward in the annulus.
U.S. Pat. No. 4,687,066 by Evans, is directed to the use of bit nozzles having openings convergingly skewed relative to the bit centerline and to each other to cause expelled drilling fluid to spin downwardly in a vortex to sweep formation cuttings from the cutting face of the rotary cones and move them to the annulus.
In U.S. Pat. No. 4,623,027 to Vezirian, nozzles are eliminated. The mud column entering the bit is divided into sectors that diverge radially outward from the bit longitudinal centerline in mud snouts that taper downward in cross section and pass vertically between the rotary rock cutters to convey drilling fluid through the bit structure in a smooth laminar flow, relatively free of turbulence and with a minimum of throttling. The mud snouts terminate in a short distance off the rolling path of the rock cutter cones. An advantage of this design is said to be that, as the high pressure fluid stream escapes through the narrow aperture between the mud snout exit and the rock surface, a very high velocity fluid sheet is formed spreading across the hole bottom surface, producing a low pressure region immediately above the rock surface sufficient to lift rock chips and send them off up the annulus toward the surface. It is further said that the pressure drop across the mud snout discharge apertures is relatively low compared to that produced by most mud nozzles, and that as a result no energy is spent in the generation of high energy fluid streams directed downward, that no hold down forces exist, and no high energy fluid streams are produced to block the entrance of the chip clearance channels in the bit periphery.
While these differing approaches to cleaning the bottom of the hole are interesting, none, other than possibly those involving generation of vapor bubbles, are directed to nozzle structure or methods of flowing drilling fluids which cause a destructive fragmenting effect on the virgin rock at hole bottom in addition to hole cleaning.